Field of the Invention
The present disclosure relates to a method of estimating antiknock properties of a multi-fuel injection internal combustion engine.
Discussion of the Background
As fuels for internal combustion engines, alcohol fuels that can be made from many agricultural products such as sugarcanes, corns, and potatoes have been attracting increasing attention. In particular, blended fuels in which alcohol fuel is blended to gasoline have become commercially available recently, and are expected to become more widespread in the future. Note that, although there are various kinds of alcohol fuels such as ethanol and methanol, the following description is provided by taking, as an example, ethanol that is the most popular alcohol fuel.
With wide-spreading of such blended fuels, studies have been also advancing on separators by which a blended fuel fed from outside is again separated into a low octane fuel and a high octane fuel inside a vehicle. Gasoline and ethanol are different from each other in various fuel physical properties such for example as an octane rating and a heat value. For this reason, in some cases, it is more preferable that a blended fuel fed from outside be again separated inside a vehicle rather than being used as it is, and then the low octane fuel and the high octane fuel thus obtained be used selectively or be again mixed and used at a different mixing ratio depending on intended purposes and conditions. The high octane fuel, in particular, is effective to suppress knocking. Thus, in a high load range, injection of a larger volume of the high octane fuel than in a low load range enables suppression of knocking.
Japanese Unexamined Patent Application Publication No. 2006-17077 describes a multi-fuel supply internal combustion engine configured to separate a fuel in a fuel tank by use of a separator to thereby produce a low octane fuel and a high octane fuel, and to supply a combustion chamber with the two kinds of fuels with different properties at a ratio depending on operation conditions, and discloses a technique of estimating the octane ratings of these two fuels. In the disclosure in Patent Document 1, an operation mode of injecting only the low octane fuel and an operation mode of injecting only the high octane fuel are provided, and the octane ratings of the low octane fuel and the high octane fuel are estimated by performing air-fuel ratio control in the respective operation modes.